Field of the Invention
The invention relates to a communication system comprising a central data processing device and a number of access stations which are coupled thereto and which are situated in different locations, and also comprising a group of external, portable, pocket-size stations, it being possible to couple an external station to an access station via bidirectional data channel, the relevant access station comprising a message generator for forming a message and an encoder for receiving the message thus formed and for encoding said message by way of a first encoding key in order to apply a first code message thus formed to the data channel, the relevant external station comprising a first decoder for receiving the first code message and for decoding this message by way of a first decoding key which is associated with the first encoding key in order to apply a first reconstructed message to a digital processing device in the relevant external station, a locally present initiation station which is coupled to the external station in an initiation state thereof comprising a generator for applying the first decoding key to the external station. The central data processing device is formed, for example, by the management device of a data bank containing medical data which may be issued to qualified users only; such a user utilizes a predetermined work station which is identified as such. This work station constitutes an external station and the access station is then informed, for example, by a front end processor which takes over given sub-tasks from a central processor. Generally, the central data processing device may be physically distributed. In that case the data channel is formed, for example, by a data network which comprises several front end processors and which can, moreover, be used for a variety of confidential as well as public tasks. However, steps must then be taken in order to inhibit unauthorized requests for confidential information in such a network. Only work stations which are identified as being authorized may utilize the confidential information and the access stations serve to separate the sheep from the goats for each category of confidential information. In a further field of application the central dating processing device acts as the manager of a credit system in which, for example, money can be dispensed upon presentation of an electronic credit card which can be coupled to the communication system. In that case it is again necessary to identify an external station as being legitimate.